Valence State in Ce-based Heavy Fermion Compounds at High Magnetic Fields

Synchrotron x-ray absorption spectroscopy of CeRh 2 Si 2 has been performed in pulsed high magnetic fields of up to 32 T. The Ce valence is found to be slightly larger (Ce 3.047+ ) than the trivalent state (Ce 3+ ) at 5 K and decrease by applying magnetic fields higher than 20 T. The magnetic fieldinduced valence decrease seems to correspond to the metamagnetic transition in the magnetization process.We also report the magnetovolume effect expected from the field-induced valence change.First we discuss the CeRu 2 Si 2 case; we analyzed the magnetic field dependence of the Ce valence in CeRu 2 Si 2 that was observed in our previous study and found that the magnetovolume effect of CeRu 2 Si 2 is quantitatively explained by the field-induced valence change.The estimated volume expansion ∆V /V by applying magnetic fields reaches 4 × 10 -3 at 40 T in CeRu 2 Si 2 .Regarding CeRh 2 Si 2 case, the lattice elongation expected from the field-induced valence decrease is ∆L/L ∼ 4 × 10 -4 that is roughly in agreement with the value in a literature at the metamagnetic transition.